The present invention involves a method and apparatus for utilizing selvage or scrap from polyacrylonitrile (PAN) film manufacturing operations. In processing polymeric films of various types for orientation by stretching the film, a certain amount of edge portions, ends and other film scrap is generated during the manufacturing process. In some production lines it is not unusual to have 10 to 30% or more selvage materials, which should be utilized in making valuable product if the manufacturing facility is to be economic.
Various processes have been devised for using marginal strips and waste products from extruded film. In U.S. Pat. No. 4,013,745 a typical prior art system reprocesses scrap by severing and recycling it to an extruder screw inlet, along with virgin polymer. The two materials are dissolved in a common solvent and fed in a unitary stream through a sheeting die to form a film. While such reprocessing techniques are feasible for certain products, they are not suitable for recycling PAN resin for use in high-performance films.
In forming a film sheet or foil of PAN resin, gas barrier properties and appearance are important qualities of the product dependent upon uniformity of composition. Where only virgin PAN resin is employed with pure solvent in constituting the extrusion mass, homogeneous solutions can be obtained without undue processing. PAN resins may be synthesized in the solvent and used without being recovered as discrete solid particles. Also, finely divided powders of acrylonitrile homopolymers and interpolymers are relatively easy to dissolve completely, due to their small particle size, usually 1 to 15 microns. When recycling scrap or selvage resin, however, it is difficult to obtain such fine subdivision by ordinary chopping, grinding or other comminution processes. Recycled resin solids may have a size and shape which render the material difficult to handle and present problems in solvent penetration during dissolution. Even with the use of auxiliary equipment, such as homogenizers, filters, etc., it is impractical to obtain completely homogeneous solutions of the recycle resin suitable for mixing with the virgin PAN feedstock. Very small amounts of undissolved resin can provide heterogeneities and film discontinuities when cast as a single layer, especially when casting a thin film. Localized stresses due to such imperfections may result in uneven stretching, pinholes or tears in the film, which are unacceptable for gas barrier service and affect appearance adversely.